Add mechanism for tests to explicitly get a default font.

[kjlubick]

Addresses #137565 (at least for Linux, which I tested locally. May need another follow-up when we try to revert this Skia change).

If we want a way to get a specific, repeatable font, we can modify this (perhaps by exposing MakeTypefaceFromName).

As Skia continues to migrate its SkFontMgr API away from having a default factory, third_party/txt/.../platform.h seems like a useful, central place to have these font things go.

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[flar]

LGTM once the license file is updated as indicated in the task failure message.

[jason-simmons]

I'll create a PR that updates the affected tests to select a specific font.

[kjlubick]

#47493 is the better fix.